Keywords
taste buds, tongue innervation, orbital cortex, piriform cortex, solitary tract and nucleus
Chapter Outline
The Perception of Flavor Involves Gustatory, Olfactory, Trigeminal, and Other Inputs, 81
Taste Is Mediated by Receptors in Taste Buds, Innervated by Cranial Nerves VII, IX, and X, 81
Taste Receptor Cells Are Modified Epithelial Cells With Neuronlike Properties, 81
Second-Order Gustatory Neurons Are Located in the Nucleus of the Solitary Tract, 82
Information About Taste Is Coded, in Part, by the Pattern of Activity in Populations of Neurons, 83
Olfaction Is Mediated by Receptors That Project Directly to the Telencephalon, 83
Olfactory Receptor Neurons Utilize a Large Number of G Protein–Coupled Receptors to Detect a Wide Range of Odors, 84
Olfactory Information Bypasses the Thalamus on Its Way to the Cerebral Cortex, 84
Conductive and Sensorineural Problems Can Affect Olfactory Function, 84
Multiple Flavor-Related Signals Converge in Orbital Cortex, 84
The Perception of Flavor Involves Gustatory, Olfactory, Trigeminal, and Other Inputs
When we eat food or drink a beverage, we have a unified perception, centered on the tongue, of some mixture of flavors. However, this unified perception actually results from the CNS combining multiple kinds of information—somatosensory inputs reflecting things like temperature, texture, and fizziness, as well as inputs about the chemical makeup of the food from three different sources. Chemicals dissolved in the mouth stimulate taste buds, providing gustatory information. Simultaneously, vapors emanating from the food reach olfactory receptor cells through either the nostrils or the oropharynx. Finally, some dissolved chemicals or vapors stimulate trigeminal and glossopharyngeal endings in the epithelial lining of the oral and nasal cavities, providing information about things like spiciness and pungency. (The latter common chemical sense persists even in the absence of taste buds and olfactory receptors.)
Taste Is Mediated by Receptors in Taste Buds, Innervated by Cranial Nerves VII, IX, and X
The tongue is covered by a series of papillae, some of which contain taste buds.
Circumvallate and most foliate papillae are innervated by the glossopharyngeal nerve (IX), fungiform and anterior foliate papillae by the facial nerve (VII). The vagus nerve gets into the act by innervating the few taste buds farther back in the pharynx (probably more important for things like coughing when something nasty gets back there than for the perception of taste). This innervation is distinct from that taking care of touch, pain, and temperature in the mouth ( Fig. 13.1 ), which is performed by cranial nerve V with some cranial nerve IX and X in the back of the tongue and pharynx.
Taste Receptor Cells Are Modified Epithelial Cells With Neuronlike Properties
Taste receptor cells utilize a variety of transduction mechanisms to detect sweet, salty, sour, and bitter stimuli.
Taste receptor cells, unlike most other receptors, are not neurons but rather are modified epithelial cells. Nevertheless, they have some very neuronlike properties: they make depolarizing receptor potentials (and many even make action potentials), which in turn increase the release of neurotransmitter (in this case, the principal transmitter is adenosine triphosphate [ATP]) onto the peripheral processes of cranial nerve fibers ( Fig. 13.2 ). Some taste receptor cells release ATP at typical chemical synapses. Others use an unusual Ca 2+ -independent mechanism in which big voltage-gated channels open and intracellular ATP just spills out into extracellular space.
Also unlike other receptors, taste receptor cells collectively use several different transduction mechanisms. These mechanisms range from very simple ones in which the Na + ions in salty foods enter the cell directly through cation channels, to more complex ones in which sweet or bitter substances initiate G protein–coupled processes.
Second-Order Gustatory Neurons Are Located in the Nucleus of the Solitary Tract
Afferents that innervate taste buds reach the brainstem with the facial nerve (from the anterior two-thirds of the tongue), the glossopharyngeal nerve (from the posterior third of the tongue), and the vagus nerve (from the epiglottis and esophagus). Like other visceral afferents (see Fig. 12.10 ), they too travel within the brainstem in the solitary tract and end in the nucleus of the solitary tract , mostly in more rostral portions ( Fig. 13.3 ).
